netfilter: remove BUG_ON() after skb_header_pointer()

Several conntrack helpers and the TCP tracker assume that
skb_header_pointer() never fails based on upfront header validation.
Even if this should not ever happen, BUG_ON() is a too drastic measure,
remove them.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_ftp.c

@@ -413,7 +413,10 @@ static int help(struct sk_buff *skb,
 
 	spin_lock_bh(&nf_ftp_lock);
 	fb_ptr = skb_header_pointer(skb, dataoff, datalen, ftp_buffer);
-	BUG_ON(fb_ptr == NULL);
+	if (!fb_ptr) {
+		spin_unlock_bh(&nf_ftp_lock);
+		return NF_ACCEPT;
+	}
 
 	ends_in_nl = (fb_ptr[datalen - 1] == '\n');
 	seq = ntohl(th->seq) + datalen;

net/netfilter/nf_conntrack_h323_main.c

@@ -146,7 +146,8 @@ static int get_tpkt_data(struct sk_buff *skb, unsigned int protoff,
 		/* Get first TPKT pointer */
 		tpkt = skb_header_pointer(skb, tcpdataoff, tcpdatalen,
 					  h323_buffer);
-		BUG_ON(tpkt == NULL);
+		if (!tpkt)
+			goto clear_out;
 
 		/* Validate TPKT identifier */
 		if (tcpdatalen < 4 || tpkt[0] != 0x03 || tpkt[1] != 0) {

net/netfilter/nf_conntrack_irc.c

@@ -143,7 +143,10 @@ static int help(struct sk_buff *skb, unsigned int protoff,
 	spin_lock_bh(&irc_buffer_lock);
 	ib_ptr = skb_header_pointer(skb, dataoff, skb->len - dataoff,
 				    irc_buffer);
-	BUG_ON(ib_ptr == NULL);
+	if (!ib_ptr) {
+		spin_unlock_bh(&irc_buffer_lock);
+		return NF_ACCEPT;
+	}
 
 	data = ib_ptr;
 	data_limit = ib_ptr + skb->len - dataoff;

net/netfilter/nf_conntrack_pptp.c

@@ -544,7 +544,9 @@ conntrack_pptp_help(struct sk_buff *skb, unsigned int protoff,
 
 	nexthdr_off = protoff;
 	tcph = skb_header_pointer(skb, nexthdr_off, sizeof(_tcph), &_tcph);
-	BUG_ON(!tcph);
+	if (!tcph)
+		return NF_ACCEPT;
+
 	nexthdr_off += tcph->doff * 4;
 	datalen = tcplen - tcph->doff * 4;
 

net/netfilter/nf_conntrack_proto_tcp.c

@@ -338,7 +338,8 @@ static void tcp_options(const struct sk_buff *skb,
 
 	ptr = skb_header_pointer(skb, dataoff + sizeof(struct tcphdr),
 				 length, buff);
-	BUG_ON(ptr == NULL);
+	if (!ptr)
+		return;
 
 	state->td_scale =
 	state->flags = 0;
@@ -394,7 +395,8 @@ static void tcp_sack(const struct sk_buff *skb, unsigned int dataoff,
 
 	ptr = skb_header_pointer(skb, dataoff + sizeof(struct tcphdr),
 				 length, buff);
-	BUG_ON(ptr == NULL);
+	if (!ptr)
+		return;
 
 	/* Fast path for timestamp-only option */
 	if (length == TCPOLEN_TSTAMP_ALIGNED

net/netfilter/nf_conntrack_sane.c

@@ -95,7 +95,10 @@ static int help(struct sk_buff *skb,
 
 	spin_lock_bh(&nf_sane_lock);
 	sb_ptr = skb_header_pointer(skb, dataoff, datalen, sane_buffer);
-	BUG_ON(sb_ptr == NULL);
+	if (!sb_ptr) {
+		spin_unlock_bh(&nf_sane_lock);
+		return NF_ACCEPT;
+	}
 
 	if (dir == IP_CT_DIR_ORIGINAL) {
 		if (datalen != sizeof(struct sane_request))